If Canon makes their cameras not compatible with new lenses, then they will loose market share because many people canot afford their lenses. I bouth a 5D MIII and am waiting for delivery but cannot but a 300 f/8! It is ridiculus that this lens price is so high. Sigma is much cheaper and I have read that is very, very good! I would probaly have to try it out before I buy it.
I think that Canon is going to feel some pressure and hopefully the Nikon thing will put some sense in to their stragety.
Actually it's the 3rd party lenses that have to be 100% compatible with Canon not vice versa.
The problem is when new bodies are introduced, Canon's responsibility is to make these bodies compatible with Canon lenses and not to care about backward compatibility with 3rd party lenses.
So the Sigma (or Tamron or Tokina, etc...) lens that works today say on a Canon 5DmkIII will not necessarily work with a possible future body 5DmkVI.
As both Tokina and Sigma claimed inability to update their old lenses to work with digital Canon bodies I do not trust them any more. This can happen again.
I know that many Canon lenses can get extremely expensive
. I get what I can.